Gas turbine engines, also known as combustion turbine engines are rotary engines that extract energy from a flow of combusted gases passing through the engine onto a multitude of turbine blades. Gas turbine engines have been used for land and nautical locomotion and power generation, but are most commonly used for aeronautical applications such as for airplanes and helicopters. In airplanes gas turbine engines are used for propulsion of the aircraft.
Gas turbine engines also usually power a number of different accessories such as generators, starter/generators, permanent magnet alternators (PMA), fuel pumps, and hydraulic pumps. All these equipment provide for functions needed on the aircraft other than moving the aircraft. For example, when the gas turbine engine is running the starter generator (S/G) produces electrical power and when the gas turbine needs to be started the S/G serves as a starting motor when provided energy from another energy source.